Page 13 - 86 human physiology part-2

Chapter 12

hepatocytes acting as unipotential stem cells. There is also some evidence of bio potential stem cells,
called oval cell, which can differentiate into either hepatocytes or cholangiocytes (cells that line bile
ducts).

The various functions of the liver are carried out by the liver cells or hepatocytes.

• The liver produces and excretes bile requires for dissolving fats. Some of the bile drains
directly into the duodenum, and some is stored in the gallbladder
• The liver performs several roles in carbohydrate metabolism:
• gluconeogenesis (the formation of glucose from certain amino acids, lactate or glycerol)
• Glycogenolysis (the formation of glucose from glycogen)
• Glycogenesis (the formation of glycogen from glucose)
• The breakdown of insulin and other hormones
• The liver is responsible for the mainstay of protein metabolism.
• The liver also performs several roles in lipid metabolism:
• cholesterol synthesis
• The production of triglycerides (fats)
• The liver produces coagulation factors I (fibrinogen), II (prothrombin), V, VII, IX, X and
XI, as well as protein C, Protein S and antithrombin.
• The liver breaks down hemoglobin, creating metabolites that are added to bile as pigment
• The liver breaks down toxic substances and most medicinal products in a process called
drug metabolism. This sometimes results in toxication, when the metabolite is more toxic than
its precursor.
• The liver converts ammonia to urea.
• The liver stores a multitude of substances, including glucose in the form of glycogen,
vitamin B12, iron, and copper
• In the first trimester fetus, the liver is the main site of red blood cell production. By the
32nd weeks of gestation, the bone marrow has almost completely taken over that task.
• The liver is responsible for immunological effects the reticuloendothelial system if the liver
contains many immunologically active cells, acting as a 'sieve' for antigens carried to it via the
portal system.

Gallbladder

The gallbladder is a pear shaped organ that stores about 50 ml of bile (or "gall") until the body
needs it for digestion. The gallbladder is about 7-10cm long in humans and is dark green in appearance
due to its contents (bile), not its tissue. It is connected to the liver and the duodenum by biliary tract.

The gallbladder is connected to the main bile duct through the gallbladder duct (cystic duct). The
main biliary tract runs from the liver to the duodenum, and the cystic duct is effectively a "cul de sac",
serving as entrance and exit to the gallbladder. The surface marking of the gallbladder is the
intersection of the midclavicular line (MCL) and the trans pyloric plane, at the tip of the ninth rib. The
blood supply is by the cystic artery and vein, which runs parallel to the cystic duct. The cystic artery is
highly variable, and this is of clinical relevance since it must be clipped and cut during a
cholecystectomy.

The gallbladder has a epithelial lining characterized by recesses called Aschoff's recesses, which
are pouches inside the lining. Under epithelium there is a layer of connective tissue, followed by a
muscular wall that contracts in response to cholecystokinin, a peptide hormone by the duodenum.

228 | Human Physiology